Expose JSON Schema ID and maturity as class methods #72

Feature description

In the spec:

{
  "$id": "https://w3id.org/ga4gh/schema/va-spec/1.1.0-snapshot.2026-06.1/base/json/Agent",
  "maturity": "trial use"
}

I imagine others will want this too. It'd be useful to expose this info through class methods, like the following:

CanonicalAllele.schema_id()
# "https://w3id.org/ga4gh/schema/va-spec/1.1.0-snapshot.2026-06.1/base/json/Agent"

Agent.maturity()
# "trial use"

It'd also be nice to expose this metadata in model_json_schema

Use case

As I have began working on the starter kit and bundle format for civicpy, I need a way to resolve the schema identifiers.

Proposed solution

Follow similar approach as ga4gh/cat-vrs-python#48

We'll need to override schema_id since VA-Spec is different and W3IDs can be found at base/aac-2017/ccv-2022/amcg-2015